FIGS. 1 and 2 illustrate a wellbore 100 extending into the earth 102. For example, the wellbore may be a natural gas well or an oil well. FIG. 1 is a cross section view and FIG. 2 is a view from the top, as if you were standing at the edge of the wellbore looking into it. Drilling the wellbore 100 creates a face 101, which is a boundary between the wellbore and the earth 102. The wellbore also includes a casing 103, which is typically a metal tubular structure. The ring-shaped space between the face 101 and the casing 103, is referred to as an annulus and is denoted as numeral 104. The relative sizes of the annulus and the casing are not to scale in the figures. It is important to note that, the wellbore 100 may include horizontal sections even though the wellbore is illustrated as only a vertical section. Distance along the wellbore from the surface is referred to herein as depth, even in horizontal sections.
It is common practice in the art to cement the casing 103 into the wellbore 100 by filling the annulus 104 with cement. This serves several purposes. It provides stability to the casing. It also prevents fluid communication between formations or reservoirs at different depths within the wellbore. For example, assume that formation 105 is a hydrocarbon-producing formation, like the Barnett Shale formation in Texas, which produces natural gas. Further assume that formation 106 is an aquifer (i.e., a formation containing water). If the annulus 104 were left empty, then it would provide fluid communication between formations 105 and 106. Water from the aquifer 106 would damage the valuable gas-producing formation 105 and gas from formation 105 would contaminate the aquifer 106.
If the cement in the annulus 104 is to serve its purpose of preventing cross-contamination between various formations and reservoirs, it is important that the cement be free of any paths, such as spaces, cracks or fissures that could provide fluid communication between such formations. For example, if the cement does not bond well to the casing 103, then a space might be left between the casing and the cement that can provide a path for fluid communication between formations.
Casing integrity can also fail if the casing becomes too thin, for example, due to wear and/or corrosion. Conversely, scale or other buildup can cause the casing to thicken. It is therefore common practice to measure several parameters concerning the condition of a wellbore casing and cement. For example, it is common practice to evaluate the quality and strength of the cement and the quality of its bond with the casing. This practice is referred to as cement bond logging (CBL). Likewise, it is common to measure the thickness of the casing of a wellbore and the internal diameter of the casing, to detect thinning and to detect places where the wellbore may become constricted.
Referring to FIG. 2, the tubulars used for the casing 103 have a particular wall thickness 107. Well servicing operators may choose tubulars having different wall thicknesses for various different situations. For reasons described below, measuring parameters of cased wellbores becomes difficult when thick casings are used. The ability to acquire accurate measurements in wells having thick casing walls is important, and thus new measurement methods are needed.